Catechin possesses two benzene rings (called the A- and B-rings) and a dihydropyran heterocycle (the C-ring) with a hydroxyl group on carbon 3. The A ring is similar to a resorcinol moiety while the B ring is similar to a catechol moiety. There are two chiral centers on the molecule on carbons 2 and 3. Therefore, it has four diastereoisomers. Two of the isomers are in trans configuration and are called catechin and the other two are in cis configuration and are called epicatechin.

The most common catechin isomer is the (+)-catechin. The other stereoisomer is (-)-catechin or ent-catechin. The most common epicatechin isomer is (-)-epicatechin (also known under the names L-epicatechin, epicatechol, (-)-epicatechol, l-acacatechin, l-epicatechol, epi-catechin, 2,3-cis-epicatechin or (2R,3R)-(-)-epicatechin).

Making reference to no particular isomer, the molecule can just be called catechin. Mixtures of the different enantiomers can be called (+/-)-catechin or DL-catechin and (+/-)-epicatechin or DL-epicatechin.

Diastereoisomers gallery

(+)-catechin (2R,3S)

(-)-catechin (2S,3R)

(-)-Epicatechin (2R,3R)

(+)-epicatechin (2S,3S)

3D view of "pseudoequatorial" (E) conformation of(+)-catechin

Moreover, the flexibility of the C-ring allows for two conformation isomers, putting the B ring either in a pseudoequatorial position (E conformer) or in a pseudoaxial position (A conformer). Studies confirmed that (+)-catechin adopts a mixture of A- and E-conformers in aqueous solution and their conformational equilibrium has been evaluated to be 33:67.[3]

Regarding the antioxidant activity, (+)-catechin has been found to be the most powerful scavenger between different members of the different classes of flavonoids. The ability to quench singlet oxygen seems to be in relation with the chemical structure of catechin, with the presence of the catechol moiety on ring B and the presence of a hydroxyl group activating the double bond on ring C.[4]

Catechin exists in the form of a glycoside.[5] Antioxidant properties can also be provided using a catechin associated with a sugar. In 1975-76, a group of USSR scientists of Kaz ssr discovered first the catechin rhamnoside using the plants of Filipendula that grow in that region. Pioneer and head of the discovery was PhD N. D. Storozhenko born in 1944. Though not thoroughly studied, the rhamnoside of catechin can enter the blood cell without breaking the outer layer.

Electrochemical experiments show that (+)-catechin oxidation mechanism proceeds in sequential steps, related with the catechol and resorcinol groups and the oxidation is pH-dependent. The oxidation of the catechol 3′,4′-dihydroxyl electron-donating groups occurs first, at very low positive potentials, and is a reversible reaction. The hydroxyl groups of the resorcinol moiety oxidised afterwards were shown to undergo an irreversible oxidation reaction.[6]

Catechins and epicatechins are found in cocoa,[11] which, according to one database, has the highest content (108 mg/100 g) of catechins among foods analyzed, followed by prune juice (25 mg/100 ml) and broad bean pod (16 mg/100 g).[12]Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), contains (+)-catechins (67 mg/kg).[13] (-)-Epicatechin and (+)-catechin are among the main natural phenols in argan oil.[14]

The biosynthesis of catechin begins with a 4-hydroxycinnamoyl CoA starter unit which undergoes chain extension by the addition of three malonyl-CoAs through a PKSIII pathway. 4-hydroxycinnamoyl CoA is biosynthesized from L-phenylalanine through the Shikimate pathway. L-phenylalanine is first deaminated by phenylalanine ammonia lyase (PAL) forming cinnamic acid which is then oxidized to 4-hydroxycinnamic acid by cinnamate 4-hydroyxylase. Chalcone synthase then catalyzes the condensation of 4-hydroxycinnamoyl CoA and three molecules of malonyl-CoA to form chalcone. Chalcone is then isomerized to naringenin by chalcone isomerase which is oxidized to eriodictyol by flavonoid 3’- hydroxylase and further oxidized to taxifolin by flavanone 3-hydroxylase. Taxifolin is then reduced by dihydroflavanol 4-reductase and leucoanthocyanidin reductase to yield catechin. The biosynthesis of catechin is shown below[19][20][21]

In rats, all plasma catechin metabolites are present as conjugated forms and mainly constituted by glucuronidated derivatives. In the liver, the concentrations of catechin derivatives are lower than in plasma, and no accumulation is observed when the rats are adapted for 14 days to the supplemented diets. The hepatic metabolites are intensively methylated (90–95%), but in contrast to plasma, some free aglycones can be detected.[28] Rats fed with (+)-catechin and (-)-epicatechin exhibit (+)-catechin 5-O-β-glucuronide and (-)-epicatechin 5-O-β-glucuronide in their body fluids.[29] The primary metabolite of (+)-catechin in plasma is glucuronide in the nonmethylated form. In contrast, the primary metabolites of (-)-epicatechin in plasma are glucuronide and sulfoglucuronide in nonmethylated forms, and sulfate in the 3'-O-methylated forms (3'OMC).[30] Catechin is absorbed into intestinal cells and metabolized extensively because no native catechin can be detected in plasma from the mesenteric vein. Mesenteric plasma contains glucuronide conjugates of catechin and 3'-O-methyl catechin, indicating the intestinal origin of these conjugates. Additional methylation and sulfation occur in the liver, and glucuronide or sulfate conjugates of 3'OMC are excreted extensively in bile. Circulating forms are mainly glucuronide conjugates of catechin and 3'OMC.[31] Another study shows that catechin undergoes enzymatic oxidation by tyrosinase in the presence of glutathione (GSH) to form mono-, bi-, and tri-glutathione conjugates of catechin and mono- and bi-glutathione conjugates of a catechin dimer.[32]

CYP1A1 (Cytochrome P450, family 1, member A1) is an enzyme implied in the metabolism of xenobiotics. Catechin decreases its expression.

SOD (Superoxide dismutase) is an enzyme that catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. Catechin increases its expression.

BAX (Bcl-2–associated X protein) is a protein of the Bcl-2 gene family. It promotes apoptosis by competing with Bcl-2 proper. Catechin increases its expression.

CASP3 (Caspase 3) is a protein that plays a central role in the execution-phase of cell apoptosis. Catechin increases its expression.

MAPK1 (Mitogen-activated protein kinase 1) and MAPK3 (Mitogen-activated protein kinase 3) are enzymes that are extracellular signal-regulated kinases (ERKs) and act as an integration point for multiple biochemical signals, involved in a wide variety of cellular processes such as proliferation, differentiation, transcription regulation, and development. Catechin seems to increase their expression.

Experiments on human Caco-2 cells show changes in the expression of genes like STAT1, MAPKK1, MRP1 and FTH1 genes, which are involved in the cellular response to oxidative stress, are in agreement with the antioxidant properties of catechin. In addition, the changes in the expression of genes like C/EBPG, topoisomerase 1, MLF2 and XRCC1 suggest novel mechanisms of action at the molecular level.[48]

It is released into the ground by some plants to hinder the growth of their neighbors, a form of allelopathy.[50]Centaurea maculosa, the spotted knapweed, is the most studied plant showing this behaviour, catechin isomers, both released into the ground through its root exudates, have effects ranging from antibiotic to herbicide. It causes a reactive oxygen species wave through the target plant's root starting in the apical meristem rapidly followed by a Ca2+ spike that kills the root cells through apoptosis.[51] Most plants in the European ecosystem have defenses against catechin, but few plants are protected against it in the North-American ecosystem where Centaurea maculosa has been introduced causing uncontrolled growth of this weed.